Balloon catheters are used in a variety of medical therapeutic applications including intravascular angioplasty. For example, a balloon catheter device is inflated during PTCA (percutaneous transluminal coronary angioplasty) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. After inflation, the pressurized balloon exerts a compressive force on the lesion thereby increasing the inner diameter of the affected vessel and improving blood flow. Soon after the procedure, however, a significant proportion of treated vessels re-narrow.
To prevent restenosis, short flexible mesh cylinders known as stents, constructed of metal or various polymers, are implanted within the vessel to maintain lumen size. Balloon-expandable stents are mounted on the periphery of the collapsed balloon portion of a balloon catheter at a diameter smaller than when deployed. During angioplasty, the balloon catheter carrying the stent is advanced through a network of tortuous blood vessels to the desired site. The balloon is inflated and expands the stent to a final diameter. After deployment, the stent remains in the vessel and the balloon is deflated, and the catheter is removed.
Although widely used, balloon catheters have significant limitations as stent delivery devices. The stent must be firmly attached to the exterior of the balloon, so that it does not become dislodged as the catheter passes through the vascular system to the target site. For this purpose, the stent is crimped to a sufficiently small diameter so that it grips the balloon. The shape of the balloon may be used to help secure the stent. Some catheter designs include sleeves that cover the ends of the stent, and stabilize it during passage through the vascular system.
The characteristics of the balloon including strength, flexibility and compliance are optimized to provide the desired performance. Nevertheless, problems may be encountered during expansion of the balloon and deployment of the stent. Frequently the stent does not cover the entire surface of the balloon. Consequently, as the balloon is pressurized, the areas of the balloon not covered by the stent expand first, causing uneven expansion and possibly deformation of the stent. In addition, after the balloon is deflated, it assumes an irregular shape, causing a relatively large effective diameter, and making retraction of the catheter from the vascular system difficult. Because of these limitations, it would be desirable to devise a mechanism for deploying a stent from a catheter that does not require a balloon.
To avoid the need for a balloon, various mechanical means to expand and deploy stents have been disclosed. U.S. Pat. No. 6,217,585 discloses a deployment device that consists of an expansion framework or cage near the distal end of the catheter. The cage consists of elongated strands coupled at opposite ends to the catheter and to a control means at the proximal end of the strands. Axial movement of the control device relative to the catheter either elongates the strands to radially collapse the cage, or axially reduces the distance between the strand ends for radial enlargement. U.S. Pat. No. 6,364,887 discloses a stent deployment device that comprises a catheter with an introducing head near the distal end of the catheter that transports the stent. On the circumference of the introducing head are radially expanding elements such as springs that, when released, press against the inside of the stent and cause it to expand. Both of these inventions have the disadvantage of requiring a fairly complex mechanism to enable the operator to control and engage the expansion mechanism without putting undue force on surrounding tissues.
U.S. Pat. No. 5,902,333 discloses a delivery system that includes a catheter that transports a radially compacted stent or other prosthesis near the distal end of the catheter. The catheter has a dilating tip that is distal to the stent mounting area. Both the proximal and distal portions of the dilating tip are gradually tapered so that the mid-portion of the tip has the largest diameter, and is slightly larger than the diameter of the radially compacted stent. As the catheter is advanced to the treatment site, the distally tapered tip may be used to gently widen the vessel lumen. After the stent is released from the catheter, the dilating tip may be passed through the lumen of the stent as the catheter is retracted, causing the tip to expand the stent to an internal diameter approximately equal to the largest diameter of the tip. Although simple to use, the utility of this device is limited because the diameter of the dilating tip must be smaller than the diameter of the blood vessel, and therefore, smaller than the optimal, final diameter of the deployed stent.
It would be desirable, therefore to provide a device and method delivering and deploying a stent or other prosthesis that would overcome these and other limitations.